Don't use THX certification as a basis for buying gear, especially if you don't even know what it means. Choose the gear on what it can or can't do based on what you need it to do and what features it has. There are two levels of certification for receivers/amps - Select and Ultra. I believe the 70x from Onkyo are Select, and the difference between the two relates mostly to the size of your room and what the amp section is certified to be capable of handling.
I agree with rnatalli, your budget is skewed considerably, as the receivers you are looking at are overkill for the speakers you are thinking about so I would rethink the allocation of funds.